AM

Amplitude Modulation or AM is the simplest form of modulation. Connecting a waveform to a VCA CV input will cause changes in the amplitude of its audio output. The controlling waveform is called the modulator and the modulated audio is called the carrier.



The resulting sound will be more complex even though only a few modules are used, but not necessarily musically pleasing. As an example, consider what will happen to the sound when the modulator is fixed at a given frequency. The Carrier produces Sum and Difference signals resulting in sidebands above and below that aren't necessarily linked to the harmonic series. When there is no harmonic relationship between any of them, though the components at the output lie at frequencies equally spaced each side of the Carrier frequency, they will be randomly scaled, resulting in enharmonic and hence dissonant sounds. Generally, enharmonicity is the result when virtually all signals are treated in this fashion. As a result, fixed-Modulator AM is mostly useful for creating aggressive and conventionally 'unmusical' sounds that change dramatically as you play up and down the keyboard. You can even control the amount of enharmonicity by raising or lowering the level of the Modulator.



But what happens when the modulator is not fixed? hwo does this affect the sounds produced by your synthesiser? to answer this, consider what happens when you're 'playing' both the Carrier and the Modulator using the same cv source to affect the frequency of both signals. If the synth is patched so that both carrier and modulator are the same base frequency, as you play the Carrier up and dwon the Keyboard, the Modulator frequency tracks the change in the Carrier frequency, so if you play a Carrier frequency one octave higher than the initial shared frequency, the Modulator frequency doubles too, and the Sum and Difference frequencies become scaled respectively. That is to say, the relationship between the Sum, Carrier and Difference signals is fixed to spacings that follow the harmonic series, i.e. the frequency may have doubled, but the shape of the waveform itself has remained the same.

Indeed, no matter what intitial frequencies you choose, the relationships between Sum, Carrier and Difference remain constant if both the Carrier and the Moculator track the keyboard equally. As a result you always obtain a consistent tone at the output. So this form of Amplitude Modulation offers a way to create complex non-harmonic timbres that change pitch normally as you play up and down the keyboard yet using far fewer modules than mixing waveforms or additive synthesis techniques.